This invention relates to television apparatus, and, more particularly, to apparatus for the separation of the luminance and chrominance components of an NTSC color television signal.
Color decoders employed in the NTSC system of color television require as a first step the separation of the luminance and chrominance components of the video signal, this function occasionally being performed by a comb filter. In a typical comb filter, the incoming video signal is separated by suitable delay lines into a main (M) line component and two adjacent line components, usually referred to as "top" (T) and "bottom" (B) signals, the main and top signals being delayed by one and two horizontal lines, respectively, relative to the bottom signal. Because the chrominance signal reverses in phase from line to line, when the main (M), top (T) and bottom (B) signals are added together in the amplitude ratios of T/4 + B/4 + M/2, the chrominance component of the video signal is cancelled thereby to derive the luminance component. By the same token, when M/2 is subtracted from the sum of B/4 + T/4, the luminance component is cancelled so as to produce the negative of the chrominance component, theoretically free of any luminance signal.
As a practical matter, however, the above-described cancellation obtains only if there have been no changes in the picture as it is "scanned" from top to bottom through the three lines; that is, cancellation of chrominance is complete only if it is assumed that the chrominance is identical on all three lines. This assumption is usually true for large areas of the picture, but should a color transition take place vertically, such as might occur with a change in hue, so that the chrominance on the top line does not have the same phase as the chrominance on the bottom and main lines, some of the chrominance signal is not cancelled, and spurious chrominance signals appear in the luminance signal. Because it is the luminance component that determines the brightness of the picture, the uncancelled chrominance component with its 3.58MHz subcarrier superimposed on the luminance signal causes a very noticeable scintillation in the display along the line of color transition, particularly in large screen television displays where every line is quite marked.
The primary object of the present invention is to provide comb filter apparatus for color television systems which removes spurious chrominance signals appearing in the luminance signal output of the comb filter, thereby to improve the quality of the television display.